There are three process of heat transfer from one place to another:
Conduction
In Conduction process heat transfer is done by direct contact of particles of matter. Energy transfer is primarily by elastic impact as in fluids or by diffusion of free electron in metals. In solids conduction is higher than that in gaseous medium or liquid medium.
Convection
In convection heat transfer is done by the movement of molecules from one part of the material to another. Convective heat transfer increase if fluid motion increases. Generally Heat Transfer due to convection can be written as
Q = h.A.(Ts-Ta)
Radiation
In radiation the heat transfer is done trough empty space. Objects having temperature above absolute zero radiate energy.
Energy radiated due to temperature difference in surrounding and object can be defined as
Q = -e.A.(T4o – T4s) where : Ts, To are surrounding and object temperature.
Newton's law of cooling
According to Newton’s law of cooling “ heat loss rate of a body is proportional to the difference in temperatures between the its surrounding and its body”.
In differential equation that law can be written as :
dQ/dt = h. A. (Tenv – T(t)) = - h.A.deltaT(t)
The Conduction Equation
By using Fourier's Law to perform a heat balance in three dimensions, the following equation can be derived relating the temperature in the system at a given point to the coordinates of that point and the time elapsed:
-k.(d2T/dx2 + d2T/dy2 + d2T/dz2) = p.Cp.dT/dt
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